The aqueous stream is typically a municipal, raw sewage sludge or an industrial waste, delivered from a sludge tank through piping to dewatering equipment, usually a belt press. The phenomenon is classified as solids-liquid separation. To aid in separating the solid matter, a flocculant is added downstream of the sludge tank to cause fine particles to collect together as much larger and denser agglomerates resulting in higher cake solids following dewatering. The flocculants are known polymers; see The Nalco Water Handbook, Second Edition Frank N. Kemmer, McGraw-Hill Book Company, 1988.
It can be readily visualized that the effectiveness of the flocculant will be proportional to the amount of clarified water downstream of the point where the flocculant is added, just before the sludge stream reaches the dewatering equipment. By measuring effectiveness, the polymer feed-rate can be adjusted for optimum treatment.
Flocculant effectiveness in two-phase aqueous suspensions or dispersions has heretofore been measured by taking the root mean square (RMS) of a voltage waveform produced by particles modulating a light source of the stream clarity. The highest voltage wave crest occurs when the beam is strongest (between particles) and lowest when the light is blocked because of an interdicting opaque agglomerate. To do this, a high intensity light beam originated by a probe on one side of the piping is transmitted across the sample stream and aimed at an opposed probe on the opposite side which incorporates an optical transducer. The transducer transforms the intensity of the beam of light into a voltage analog waveform. An RMS-to-DC converter chip then extracts the RMS voltage of the wave and emits a proportional D.C. voltage as a value proportional to the degree of flocculation. U.S. Pat. No. 4,752,131 is typical.